I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for gating the transmission of signals in a wireless communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech is by the use of variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in U.S. Pat. No. 5,414,796, entitled xe2x80x9cVARIABLE RATE VOCODERxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when said speech encoding is providing speech data at a maximum rate. When a variable rate speech coder is providing speech data at a less that maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. In the above mentioned patent application a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.
In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore, message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. Pat. No. 5,659,569, entitled xe2x80x9cDATA BURST RANDOMIZERxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
In a communication system that provides data using a QPSK modulation format, very useful information can be obtained by taking the cross product of the I and Q components of the QPSK signal. By knowing the relative phases of the two components, one can determine roughly the velocity of the mobile station in relation to the base station. A description of a circuit for determining the cross product of the I and Q components in a QPSK modulation communication system is disclosed in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUITxe2x80x9d, assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in copending U.S. Pat. No. 08/886,604, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The present invention is a novel and improved method and apparatus for controlling the transmission power in a plurality of base stations simultaneously communicating with a mobile station in soft handoff. In a power controlled communication system where multiple transmitters at different physical locations are employed to transmit the same signal to a given receiver, the receiver measures the quality of the composite received signal from all transmitters and feeds back this observed quality to the transmitters. In the exemplary implementation, the feedback is a single stream of up or down commands from the receiver that is received by all transmitters involved. However, the reliability of the reception is not uniform across transmitters. Further, the reliability of the feedback to any given transmitter can change over time. As a result,the transmitters follow their individually received feedback and transmit at different power level at the same time.
It is usually advantageous to align the transmit power levels for a given receiver from all participating transmitters according to a desired pattern. For example, the transmitters can also send a fixed pilot channel at the same power level. Equating the traffic channel transmit levels imply the same traffic to pilot ratios across transmitters and the best maximal ratio combining can be achieved at the receiver by taking the inner product of the pilot and traffic. Another example is when the transmitters transmitter have different maximum power and send out pilot channel at different power levels. In this case, the alignment of traffic channel power means the transmitters set their traffic transmit levels proportional to their pilot levels. This also achieves the same traffic to pilot ratios across all transmitters. Yet another example of transmit power level alignment is a system where there is a desired transmit power level pattern based on the signal-to-noise ratio or SNR from each transmitter to the receiver. If transmitter 1""s pilot SNR is twice that of transmitter 2""s, then the traffic transmit level from transmitter 1 should be twice that of transmitter 2""s. This transmit level pattern can be followed by all transmitters as the overall transmit level changes according to the feedback command. The present invention proposes a series of methods that can be used to align the transmit power of transmissions to a mobile station in soft handoff.
In the first exemplary embodiment, the transmitters are attached to a separate control unit through communication links. This control unit receives the power control commands received at each base station and optionally a quality indicator for each command from each base station. The control unit then derives the most likely command stream and send that to the base stations. The base stations use this to override the transmit power level they were using, or use it plus the feedback commands it received during the processing and relaying of this most likely command to determine the transmit level.
In the second exemplary embodiment, the control unit periodically receives the final or average transmit level in a period and an aggregate quality measure for the feedback during a period from each of the transmitters. The control unit determines the aligned power level and transmits a message indicative of the aligned power level to the transmitters.
In the third exemplary embodiment, the transmitters send the control unit a message indicative of the transmit power of transmissions to the receiver. The control unit determines the aligned transmit power based on the current transmit power. For example, the control unit can inform all transmitters of the average value of the transmit traffic to pilot ratios it most recently received from the transmitters if the desired transmit power level pattern is to have all traffic to pilot ratios identical. The transmitters would then make corrections to its current transmit level by the delta between what it received from the control unit and what it actually used at the time corresponding to that.
In the fourth exemplary embodiment, transmitters send the control unit a message indicative of the transmit power of transmissions to the mobile station. The control unit determines the aligned transmit power based on the current transmit power. The correction is done only when the transmit levels diverge beyond a certain threshold from the desired pattern. This thresholding can lessen the backhaul loading. Also, the correction can be less than what is needed for full alignment to lessen the impact on the closed loop and outerloop operation. For example, assume the desired alignment pattern is to have all transmitters to transmit at identical traffic to pilot ratios, when the difference between the highest and the lowest transmit levels is less than X dB, the control unit sends no correction to the transmitters. (Or, it sends the individual corrections or the common desired level but the transmitters do not correct if the needed correction is less than Y dB.) When there is a difference equal to or larger than X dB, the control unit computes the average transmit traffic to pilot ratioand forwards it to the transmitters. The transmitters computes the necessary correction and applies it. Alternatively, the control unit can compute the amount of correction for all the transmitter and send them individually to the transmitters where they are applied. The correction can be a fixed percentage of the necessary amount to bring all transmitters together. Or the correction can be a fixed step, say Z dB; or a fixed percentage of the necessary step, say W%, regardless of what is necessary to get all transmitters aligned. In addition, this correction can be applied gradually over time. The full desired correction is achieved right before the next correction is received from the control unit.
In the fifth exemplary embodiment, similar to the previous two embodiments, the correction can be derived from the quality indicator of the feedback at each transmitter. For example, this quality indicator can be based on the strength of the reverse link pilot or the amount of time it is in lock at each of the transmitters. The quality indicator can also be based on reverse link frame erasures at the transmitters. It can also depend on the signal to noise and interference ratio (Ec_pilot/Nt summed over all fingers locked on a given BTS) for each transmitter at the receiver. That is, when the control unit examines the transmit levels from the transmitters, the transmit levels or traffic to pilot ratios used by the transmitters that have better feedback quality, and by those transmitters whose signal is stronger at the receiver should be emphasized. The above will improve the xe2x80x98correctxe2x80x99 transmit level since the correlation between forward link and reverse link is usually positive, and a clear feedback indicates a stronger forward link at the receiver. Therefore, if the transmit level at the transmitter that has a better feedback quality is modified the least, the impact on the total received Eb/Nt at the receiver would be less and the impact on the closed loop and outer loop is minimized.
In the sixth exemplary embodiment, the transmitters and/or the control unit apply a soft mapping between the received feedback strength and the amount of transmit level adjustment. That is, the step size in the adjustment is a real number whose value depends on the value of the feedback command signal to noise ratio. A threshold can be set so that when the signal to noise ratio of the feedback is too low, the power control step size is zero. Additionally, when the feedback receiver at a transmitter is out of lock and no feedback SNR can be measured, there will be no corresponding adjustment for the transmit level. If a control unit has access to the quality of the feedback commands at the transmitters, it can use the same soft mapping to determine the most likely commands (for the first exemplary embodiment) or the most likely transmit level or traffic to pilot ratio based on the most recent feedback quality (for the second exemplary embodiment).